Well, in the context of this game, the races dont exactly like each other all that much. Considering that, it makes since why mass genocide is the only option. Really it is human nature. Humans will do just about anything to survive.
True, races don't like each other that much, but even for strategic and tactical purposes, sometimes I'd rather take a planet mostly unscathed, with its planetary developments intact or with minor damages, and with a population to pay taxes, than take a floating rock, devastated by orbital bombing.
In fact, right now the effects of massive orbital bombing are rather weak, if you ask me. One would expect that when a planet is bombed into oblivion from orbit, nuclear fallout, wheather destabilization, etc. would definitely hamper further re-colonization attempts...
What I'd like to see is the option for a planetary invasion, coupled with some sort of planet quality loss if you decide to bomb anyway, and maybe an intermediate solution consisting of selective bombing, for instance.